1. Field of the Invention
The present invention relates to an improved biological filter system whose function consists of a physical filtration by granular media (gravel, sand, special media, etc.), a biological decomposition of organic substances, an oxidation of metal substances, and an aeration of increasing dissolved oxygen. The improved biological oxidation filter system could be used as a pretreatment process of a water supply and main or post process of wastewater treatment.
2. Description of the Background Art
Recently, water consumption has been sharply increased due to the population growth and the accelerated industrialization and urbanization. Also, discharge rates of various domestic sewage, industrial wastewater and livestock wastewater have been increased, thereby contaminating more streams and lakes. Stream contamination destroys the ecosystem, increases the cost of water purification by deteriorating the quality of a water source, and even paralyzes water treatment facilities.
A slow sand filter removes a slight amount of ammonium, manganese, bacteria and odorant from the raw water through the biological decomposition. However, the slow sand filter cannot remove the contaminants to the large extent because it does not utilize coagulation and flocculation process. On the other hand, in a rapid sand filter system used to substitute the slow sand filter, suspended materials in the raw water are agglomerated in a flocculation process, settled in a sedimentation basin and sieved through a granular media consisted of sand and gravel. Although the performance of the rapid sand filter is better than the slow sand filter for most water sources, it also has a limitation based on concentration and species of the contaminants in the raw water. Especially when algae blooms due to the eutrophication of water in a reservoir or in a stagnated stream, the rapid sand filter lose its function rapidly.
In order to produce a drinking water of good quality from the raw water of poor quality, there is a strong demand for an advanced pretreatment process which can improve reliability of the conventional water treatment by reducing the pollutants in the raw water. Thus, a research has been conducted on the biological pretreatment process relating to the present invention.
Most biological processes promote natural purification activity of microorganisms in a reactor at high efficiency. A submerged honeycomb media, a rotating disc and a biological contact filter system are the known biological techniques. However those known biological techniques are in lack of reliability and adaptability if the temperature is low or the quality of raw water varies widely. In the biological process, organic substances, nitrogen, phosphorous and the nutrients in water are ingested by the microorganisms existing in a biological slime resulting in purification of water. However, it is difficult to raise the biological and chemical reactions of the microorganisms in the biological slime to the extent to remove the pollutants completely. The purification efficiency are varied according to the temperature and the quality of the raw water.
In a biological process, it is important to maintain a suitable growth environment for the microorganisms even though the quality of the raw water flowed into a processing unit is fluctuated. For a suitable growth environment, the organic substances as well as a Nitrogen, inorganic phosphorous, and a slight amount of metals are required. If any of the necessary components is deficient, a nutrient balance and the growth environment of the microorganisms are destroyed resulting in failure of the biological process. In addition to the necessary nutrients, the dissolved oxygen is sufficiently required for the oxidation decomposition of the organic substances or the oxidation of ammonia nitrogen by the microorganisms. In this regards, efforts have been made to increase the dissolved oxygen by aeration. Aeration is successful to some extent in the basin packed with submerged honeycomb media. However, there is a technical difficulty in the gravity filters such as flooding and a negative pressure development making the aeration method hardly implicated.
Primary objective of the present invention is to provide a biological filter system which can be utilized efficiently not only as a pretreatment for drinking water production but as a part of processes for wastewater treatment in large scale.
The biological filter system of present invention includes; a filter unit receiving a filter medium providing an enhanced growth environment for microbial flora; an underdrain unit collecting water during filtration and distributing air and water uniformly during backwashing; an air and/or oxygen supply unit to increase dissolved oxygen in the water to be used by the microbial flora in the filter medium, and to be used in the oxidation process of metal species in raw water; an air and water backwashing system supplying air and water under strictly controlled sequence to clean the media most efficiently; and troughs maintaining the constant water level during the backwashing, and draining the water rising over that level.
The most advanced characteristics of the biological filter system of the present invention is the configuration of system. The system consists of the medium packed lower layer and the water storing upper layer. The specific down-flowing water and up-flowing air arrangement provides sufficient dissolved oxygen in the medium resulting in the higher attached microorganism population for higher removal rates of organic materials. Furthermore the countercurrent air and water flow prevents clogging of the water path at the top layer of the filter where the most suspended matters in the raw water deposit. That causes very slow headloss development providing prolonged filter run length. The microorganisms washed out to the upper water storage layer digests organic matters in the raw water reducing organic load to the filter.